Computer printer technology has evolved to a point where high-resolution images can be transferred on to various types of media, including paper. One particular type of printing involves the placement of small drops of a fluid ink onto media surfaces in response to a digital signal. Typically, the fluid ink is placed or jetted onto the surface without physical contact between the printing device and the surface. Within this general technique, the specific method that the ink-jet ink is deposited onto the printing surface varies from system to system, and can include continuous ink deposit or drop-on-demand ink deposit.
With regard to continuous printing systems, inks used are typically based on solvents such as methyl ethyl ketone and ethanol. Essentially, continuous printing systems function as a stream of ink droplets that are ejected and directed by a printer nozzle. The ink droplets are directed additionally with the assistance of an electrostatic charging device in close proximity to the nozzle. If the ink is not used on the desired printing surface, the ink is recycled for later use. With regard to drop-on-demand printing systems, the ink-jet inks are typically based upon water and glycols. Essentially, with these systems, ink droplets are propelled from a nozzle by heat or by a pressure wave such that all of the ink droplets ejected are used to form the printed image.
There are several reasons that make ink-jet printing a popular way of recording images on various media surfaces, particularly paper. Some of these reasons include low printer noise, capability of high-speed recording, and multi-color recording. Additionally, these advantages can be obtained at a relatively low cost to consumers. However, though there have been great improvements in ink-jet printing, accompanying these improvements are increased consumer demands such as higher speeds, higher resolution, full color image formation, increased image durability, etc. As new ink-jet inks are developed, there have been several traditional characteristics to consider when evaluating the ink in conjunction with printing media. Such characteristics include edge acuity and optical density of the image on the surface, dry time of the ink on the substrate, adhesion to the substrate, lack of deviation of ink droplets, presence of all dots, resistance of the ink after drying to water and other solvents, long term storage stability, and long term reliability without corrosion or nozzle clogging. Though the above list of characteristics provides a worthy goal to achieve, there are difficulties associated with satisfying all of the above characteristics. Often, the inclusion of an ink component to address one of the above attributes prevents another being met. Thus, most commercial inks for use in ink-jet printers represent a compromise, in an attempt to achieve adequate performance in all of the above listed attributes.
Ink-jet inks are either dye- or pigment-based. Dye-based ink-jet inks generally, but not always, use water-soluble colorants. As a result, such dye-based inks are usually not always water fast. Prints made from these inks tend to undergo color change over time, or fading, when exposed to ambient light and air. The media surface can play a key role in the fade properties and wet fastness of an image in that for a given ink, the degree of fade and wet fastness can be highly dependent on the chemistry of the media surface. Therefore, for optimum performance, many ink-jet inks often require that an appropriate media be selected in accordance with the application, thus, reducing the choice of media. In the case of pigmented inks, it is the dispersed colorant particles that produce color. Often the line quality of prints produced by pigment-based inks is superior to that of dye-based inks. When a printed image is made with pigmented inks, solid colorant particles adhere to the surface of the substrate. Once the ink vehicle evaporates, the particles will generally not go back into solution, and are therefore more water fast. In addition, pigmented inks are often much more fade resistant than dye-based inks. Though pigmented inks, in some areas, exhibit superior performance, dyes in general produce inherently more color saturated and more reliable inks. Thus, dye-based inks have been more often used in applications where fade resistance is not primarily important.
In order for the ink-jet industry to effectively compete with silver halide photography, it is important that ink-jet prints must improve their image fade resistance. In other words, enhanced permanence of images has become important to the long-term success of photo-quality ink-jet ink technologies. According to accelerated tests and “industry standard” failure criteria, photographs have typically been known to last about 13 to 22 years under fluorescent light exposure. There are now even systems with published values of 19 to 30 years. The best dye based ink-jet printers produce prints that last for much less time under similar conditions.
A few categories of photographic ink-jet media are currently available: polymer coated media, clay coated media, and porous coated media. It is the polymer based type that produce the best known images, e.g. longest lasting, mentioned above. However, this category of media is generally inferior in dry time and wet fastness relative to porous coated media. On the other hand, image fade resistance and humid fastness of the porous coated media is generally lower than that of its polymer-based media counterpart. Therefore, there is a great desire to improve the image permanence of ink jet ink images on porous coated media, particularly with respect to alumina based coatings.